1. Field of the Invention
The present invention relates to a rotary-cam type reclining device suitable for use in automobile reclining seats, and more specifically to a rotary-cam type seat reclining device lockable by way of a rotary motion of the rotary cam mechanically linked to a reclining lever to ensure forward-and-backward angular adjustment of a seat back relative to a seat cushion.
2. Description of the Prior Art
In recent years, there have been proposed and developed various rotary-cam type reclining devices. A typical rotary-cam type reclining device includes a base fixedly connected to a seat cushion and having a recessed portion formed with two opposing parallel side walls, an arm fixedly connected to a seat back and rotatably supported by the base via a pivot shaft, two or more toothed blocks each slidably guided in the two opposing parallel side walls in the base, a reclining lever integrally connected to the pivot shaft, and a rotary cam usually fixedly connected to the pivot shaft of the reclining lever and acting to mesh or unmesh the toothed blocks with or from the inner toothed portion of the arm by way of rotation of the rotary cam. One such conventional rotary-cam type reclining device has been disclosed in Japanese Patent First Publication No. 6-125821 (corresponding to French Patent Application No. 91 02590 filed on Mar. 5, 1991). A conventional rotary-cam type reclining device disclosed in the Japanese Patent First Publication No. 6-125821 includes a stationary flange portion, often called a base, fixedly connected to a seat cushion, a rotatable flange portion, often called an arm, fixedly connected to a seat back, and a rotary cam. The rotatable flange portion is formed with an inner toothed portion on its inner periphery. On the other hand, the stationary flange portion is formed with a plurality of cup-like recessed portions, each radially guiding and slidably accommodating therein a toothed block. A plurality of block pushers are interposed between the rotary cam and the respective toothed blocks. Each of the toothed blocks can be meshed with or unmeshed from the inner toothed portion of the arm via radially outward sliding motion of each of the block pushers being in cam-connection with the rotary cam. One rotational movement of the rotary cam, equivalent to a pull-up motion of the reclining lever, produces a radially inward sliding movement of each block pusher and the inner toothed portion of the arm is unmeshed from the outer toothed portions of the toothed blocks, thus allowing an angular adjustment of the rotatable flange portion relative to the stationary flange portion. If the rotary cam is rotated in the other rotational direction together with the reclining lever with the seat back set at a desired angular position determined by the vehicle occupant, the outer toothed portions of the toothed blocks are brought again into meshed-engagement with the inner toothed portion of the rotatable flange portion by virtue of a radially outward sliding motion of each block pusher, resulting from the other rotational movement of the cam. Assuming that an excessively large external force such as impact force is applied to the rotatable flange portion connected to the seat back under such a fully-locked condition of the device where the toothed blocks are meshed with the rotatable flange portion, the rotatable arm is heavily loaded in a certain rotational direction and as a result a relatively large bending moment and a shearing force are produced in the toothed blocks which may be subjected to an excessively large load, transmitted from the rotatable flange portion via the plural meshing pairs to the toothed blocks. In case of application of excessively large impact force, each of the toothed blocks could be brought into collision-contact with the associated side walls of the cup-like recessed portions, with the result that a bearing pressure or bearing stress, acting on the side walls of the cup-like recessed toothed-block guiding portion, may be abruptly risen or there may be result in stress concentration on the side walls. This results in undesired deformation of the side walls of the cup-like recessed portion of the stationary flange portion. As is generally know, it is important to lighten the entire weight of the device, while insuring a required mechanical strength of the device. Additionally, to insure a smooth sliding motion of each toothed block within the cup-like recessed guide portion, there is less friction between the side wall of the recessed guide portion and the side wall of the toothed block, in sliding-contact and there is less play between the two opposing side walls. To avoid undesired deformation of each toothed block and a rotary cam in case of application of an excessively large load, it is necessary to properly increase the thickness in each side wall of the recessed guide portion and/or a thickness in each toothed block, to increase the mechanical strength of the reclining device. Alternatively, to enhance the mechanical strength, a material having a high mechanical strength, such as a high-strength steel, can be used. To lighten the device and reduce the production costs, the previously-noted two methods for the mechanical-strength enhancement is unpreferable.